The long-term goal of this project is to illuminate the mechanisms of p53 tumor-suppressor function in normal and stressed conditions in an intact animal and to identify components with which it collaborates. >50% of human tumors carry lesions p53 and many others are indirectly debilitated for p53 function. Thus, a critical step in the genesis of most cancers is disruption of p53 activity. p53 integrates cellular responses to a variety of stresses in part by regulating target genes that mediate cell cycle progress and apoptosis. As a springboard for the comprehensive analysis of p53 function in a genetically tractable system, we identified cep-1, a non-redundant C. elegans homologue of human p53. We found that CEP-1 is essential for DNA- damage-induced apoptosis in the germline, normal meiotic chromosome stability, and resistance to hypoxia. In the proposed studies, cep-1 mutants will be analyzed to test whether CEP-1 is required in the soma, which of its in vivo requirements involve its interaction with target DNAs, and whether a truncated form acts as a dominant negative to interfere with binding by wild-type CEP-1. The subcellular localization and regulation of CEP-1 expression will be assessed to determine whether CEP-1 is present throughout the soma and/or germline, whether its tissue or subcellular distribution changes under varied conditions of stress, and whether it is transcriptionally and/or post- transcriptionally responsive to stress. The comprehensive set of CEP-1 gene targets will be investigated to test whether CEP-1 functions both to activate and repress a large set of genes, including apoptotic regulatory genes, and second, whether the set of CEP-1-regulated genes changes in response to stress. The hypothesis that CEP-1/p53 collaborates with other pathways in mediating stress-response, chromosome stability, and viability, will be tested by identifying mutations that enhance and/or suppress defects in cep-1 function. The newly identified genes will help not only provide a base for genetic analysis of genes essential for p53 to execute its in vivo function, but also to reveal potential therapeutic and diagnostic targets for anti- cancer therapies in humans.